1. Field of the Invention
The preferred embodiments are directed to wind turbine generators, and more particularly, electrical architecture for such generators that facilitates operation during grid faults and low voltage events.
2. Description of the Prior Art
Worldwide capacity of wind-powered generators makes up less than 1% of world-wide electricity use. Nevertheless, although still a relatively minor source of electricity for most countries, wind power generation more than quadrupled between 1999 and 2005. For example, wind power is used in large scale wind farms to supply national electrical grids, but is also used in small individual turbines for providing electricity in isolated locations.
As the penetration of large scale wind turbines into electric power grids continues to increase, electric system operators are placing greater demands on wind turbine power plants. Specifically, wind turbine power plants must provide a specified amount of reactive power and deliver fault clearing current in the event of grid faults and the accompanying voltage sags. Low voltage ride through requirements first proposed by German electric transmission operators E.ON and VE-T in 2003, and recently adopted in the U.S. via FERC order 661 is presented in FIG. 1. In this regard, these adopted regulations address an issue with conventional wind turbine generators—voltage sags. That is, during extreme voltage sags at the point of common coupling (PCC), also referred to as “collector”, high per unit currents and shaft torque pulsations occur. These transients can place stringent demands on conventional doubly fed induction generator (DFIG) based wind turbines.
Specifically, as illustrated in FIGS. 2A-2D, the onset of a voltage sag, shown as a 1.0 per unit voltage drop in FIG. 2A, can result in significant stator flux oscillations, as shown in FIG. 2B. Moreover, the voltage sag can also cause substantial oscillations in rotor phase current. These oscillations diminish as the PCC voltage remains level, but reoccur when the PCC voltage returns to its pre-sag levels, as illustrated in FIG. 2C. In addition to stator flux and rotor current oscillations, a drop in PCC voltage can place an increasing torque on the rotor, as shown in FIG. 2D, that can result in premature failure of the wind turbine.
The difficultly in surviving severe voltage sags is a direct result of the properties of the semiconductor power conversion architecture customarily found in DFIG wind turbines. Conventional DFIG wind turbines have a semiconductor AC/DC/AC conversion chain that is used to condition the power delivered from the rotor circuit for frequency and amplitude compatibility with the grid. During extreme PCC voltage sags, very high currents are induced in the rotor circuit, as illustrated in FIG. 2C, which can damage the rotor side converter and cause undue fatigue on the gearbox.
In a conventional DFIG wind turbine 10, schematically shown in FIG. 3A, a blade arrangement 12 is driven by a gearbox 14 coupled to a DFIG circuit 16 that includes a generator 18. The stator windings 20 of the generator 18 are connected to the grid PCC 22 and are excited at the grid frequency. The rotor windings 24 of the generator 18 are connected to the AC side of a DC/AC converter 26, called the machine side converter (MSC). The AC side of a second DC/AC converter 28, commonly referred to as the grid side converter (GSC), is connected in parallel with the machine stator windings 20 and PCC 22. A transformer 30 enables voltage and current compatibility between the GSC 28, stator windings 20 and the PCC 22. The DC ports of each converter 26, 28 are connected to form a DC link 32, enabling power flow between them. A capacitor 34 stabilizes the DC link voltage. Each of the three phase DC/AC converters 26, 28 is composed of a set of IGBTs and diodes in a bridge configuration. The IGBTs are switched at very high frequency to synthesize a set of three phase AC voltage waveforms.
As shown in the simplified circuit diagram of FIG. 3B, a high bandwidth PI regulator 25 is implemented to control the rotor current via the MSC 26. With proper reference alignment, one component of the rotor current is proportional to torque; the other component of rotor current supplies reactive power to the DFIG 30. Likewise, a high bandwidth PI regulator 27 on the GSC current yields control of the DC link voltage and reactive power delivered to the grid.
One benefit of this DFIG configuration 16 is that for a practical rotor speed range, the AC/DC/AC conversion path only processes a fraction of the total electrical power delivered to the grid. The bulk of the power flows directly from the stator windings 20 to the PCC 22. As a result, the losses due to the semiconductor conversion path are minimized to produce a high net efficiency. Also, the two AC/DC converters 26, 28 can be sized for a fraction of the total electrical output power, resulting in lower investment costs and less mass installed in the nacelle.
Optimum wind energy capture is typically achieved by specifying a desired torque based on the wind speed from a look-up table. At low to intermediate wind speeds, the torque profile targets the optimum tip-speed ratio for the turbine blades. At high wind speeds the torque command rises sharply to limit the rotor speed to approximately 1.2 per unit. Rotor shaft over-speed is prevented from a regulator that pitches the turbine blades to reduce the coefficient of performance.
However, as noted above, conventional DFIG configurations have difficulty in tolerating PCC voltage disturbances. Referring again briefly to FIG. 3A, one proposed solution is to use the MSC 26 to try and reduce the magnetic field in the DFIG in response to a voltage sag. This approach has not been shown to survive voltage sags below 20% of nominal PCC voltage and requires increasing the size of the MSC. In addition significant torque spikes and oscillations occur during the voltage sag event.
Referring now to FIG. 3C, one proposed solution is the use of a three phase SCR 29 and resistor crowbar circuit 31 provided in parallel with the MSC 26 which is capable of riding through a PCC voltage sag down to 15% of nominal voltage. This approach may result in a highly oscillatory current response and compromised control of shaft torque. Analysis of a similar topology found that the rotor shaft endures torque roughly 3 to 4 times rated, the stator current is largely unchanged, and consumes reactive power from the grid during the sag event. In a further proposed modification, a pulse width modulated IGBT bridge for the SCRs in the rotor crowbar circuit has been suggested to improve voltage sag response.
Referring now to FIG. 3D, a DFIG system with three pairs of anti-parallel SCRs 33 in series with the stator connection to the grid, in addition to SCRs 29 and resistor crowbar 31 in parallel with the MSC, has also been suggested. At the inception of the voltage sag, the stator series SCRs 33 disconnect the DFIG stator terminals from the grid, while the GSC remains connected to service the grid fault current, up to its limit. The stator terminals are reconnected after the MSC demagnetizes the DFIG and resynchronizes it to the new PCC voltage, typically within 20 ms. This approach is believed to produce peak stator current and shaft torque of less than 2 per unit during sag events. Moreover, the rotor crowbar 11 remains for emergency backup protection. One of the drawbacks of this approach is that it does not provide ride-through during the voltage disturbance (i.e., the machine disconnects and does not serve the grid) and therefore does not comply with the ride-through standards.
In summary, modifications to the conventional DFIG architecture for ride-through have been proposed that include, for example, using the MSC to control the magnetic field in the DFIG, the use of a clamping circuit connected to the rotor and stator side semiconductor switches to enable brief disconnection and reconnection of the wind turbine upon inception of the sag event. However, such conventional modifications to the doubly fed induction generation (DFIG) architecture for low voltage ride-through result in limited control of the turbine shaft and grid current during fault events, thus leaving the system highly susceptible to being catastrophically damaged. A new electrical architecture for DFIG wind turbines with improved low voltage ride-through is needed.